The Tektronix 067-0500-00 (also known as 3M1) is a calibration fixture for adjusting the normalizing capacitor behind the plug-in connector in 560-series scopes.
It consists of a Schmitt trigger that is driven from a single-ended input coming from a BNC connector on the front panel. The Schmitt trigger produces balanced output signals with controlled source impedance. The normalizing capacitor is adjusted using a procedure similar to adjusting probe compensation. When adjusted correctly, the voltage division ratio formed by the plug-in's output impedance and the mainframe's input impedance should be a real value which does not change across the specified frequency range of the scope. After the scope is normalized using the 067-0500-00, other plug-ins can be adjusted to produce clean transient response in the normalized scope.